DRAM power bus control

ABSTRACT

A dynamic random access memory (DRAM) is provided that has separate array and peripheral power busing to isolate array noise from peripheral circuits such as delay lock loops during row activations and read/write memory operations. A switch connects the array power bus to another separate power bus for a limited period of time during a DRAM refresh cycle to provide additional current to the DRAM arrays. The switch disconnects the array power bus from the other power bus preferably before the end of the refresh cycle.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division of U.S. patent application Ser. No. 10/933,073, filedSep. 1, 2004, now U.S. Pat. No. 7,023,756 which is a division of U.S.patent application Ser. No. 10/227,468, filed Aug. 23, 2002, now U.S.Pat. No. 6,795,365, which is hereby incorporated by reference herein inits entirety.

BACKGROUND OF THE INVENTION

This invention relates to dynamic random access memories (DRAMs). Moreparticularly, this invention relates to power distribution on DRAMs.

A DRAM is a form of semiconductor random access memory (RAM) commonlyused as main memory in computers and other electronic systems. DRAMsstore information in arrays of integrated circuits that includecapacitors. Because capacitors lose their charge over time, they need tobe regularly recharged. This regular recharging is performed during“refresh” cycles.

DRAMs also include other circuits and devices, known as peripherals,that support memory read and write operations as well as other DRAMfunctions. High speed DRAMs, known as synchronous DRAMs (SDRAMs), useclocks to synchronize control and data signal transfers and includeperipherals known as delay lock loop (DLL) circuits to maintain thatsynchronization.

DLL circuits, however, are susceptible to power and ground bus noisetypically generated by the memory arrays during row activations and datareads and writes. This susceptibility can cause the DRAM to losesynchronization. Loss of synchronization causes timing problems that canresult in the wrong data being read from or written to memory, thusadversely affecting data integrity throughout a computer or othersystem.

One known solution is to isolate the power and ground buses connected tothe DLL and other synchronization control circuits from the power andground buses connected to the DRAM arrays. This can be done by runningseparate power and ground buses to peripheral circuits from one set ofchip power and ground input/output (I/O) pads and running separate powerand ground buses to the arrays from another set of chip power and groundI/O pads. Each chip power and ground pad is connected to an externalvoltage. Thus, rather than have a network of power and ground busescommonly connected to all respective power and ground pads, separate andisolated power and ground buses are connected to respective subsets ofthe DRAM chip's power and ground pads.

Such power distribution, however, results in less available current perseparate power bus, because each bus has fewer pads connected to it fromwhich to draw current. Each power pad can supply only a limited amountof current. This can adversely affect the DRAM arrays during refreshcycles when significantly increased amounts of current are needed torecharge the capacitors. With less current available, the capacitors maynot fully charge. This can decrease the time that a capacitor retainsthe correct stored value, thus resulting in either a loss of stored dataor more frequent refresh cycles. More frequent refresh cycles result inreduced DRAM performance, because read/write operations cannot beperformed during refresh cycles.

In view of the foregoing, it would be desirable to providesynchronization control circuits with power and ground busing havinginconsequential, if any, noise generated by memory arrays during rowactivations and read/write operations while still providing the arrayswith sufficient current during refresh cycles.

SUMMARY OF THE INVENTION

It is an object of this invention to provide synchronization controlcircuits with power and ground busing having inconsequential, if any,noise generated by memory arrays during row activations and read/writeoperations while still providing the arrays with sufficient currentduring refresh cycles.

In accordance with this invention, power distribution on DRAM chips isprovided such that DLL and other peripheral circuits are supplied withpower and ground substantially free of any noise generated by array rowactivations and read/write operations. This power and ground is suppliedvia dedicated power and ground pads not used by the arrays duringnon-refresh operations. The DRAM power distribution also suppliessufficient current to the arrays during refresh cycles via a switchableconnection between array power busing and power busing connected to oneor more power pads also not used by the arrays during non-refreshoperations.

In a preferred embodiment of the invention, a DRAM chip has array powerbuses that provide a regulated voltage to the arrays, peripheral powerbuses that provide a regulated voltage to DLL and other peripheralcircuits, and a third power bus that provides an unregulated voltage tostill other peripheral circuits. The peripheral and third power busesare connected to one or more power pads other than the power padsconnected to the array power buses. The power pads connected to theperipheral and third power buses can be the same. Preferably, theunregulated voltage provided by the third power bus is higher than theregulated voltages provided by the array and peripheral power buses. TheDRAM chip further includes a switch that connects the array power bus tothe third power bus. The switch closes for a finite period of timepreferably at the start of each refresh cycle to provide additionalcurrent to the arrays. The switch opens preferably well before or atleast by the end of the refresh cycle.

In a more preferred embodiment of the invention, the switch is ap-channel FET (field effect transistor) with its source connected to thethird power bus and its drain connected to the array power bus. A signalpulse applied to the FET's gate turns it ON (i.e., renders itconductive) for a momentary period of time preferably at the start ofeach refresh cycle. The third power bus then supplies needed currentthrough the conductive p-channel FET to the DRAM arrays. The DLL andother peripheral circuits are not affected by this because no reading orwriting of data occurs during refresh cycles, thus the temporaryconnection between the array and third power bus has no adverse affecton synchronization or data integrity.

The invention advantageously provides DLL and other peripheral circuitswith noise isolation during DRAM row activations and data reads andwrites while providing increased current during at least a portion ofeach DRAM array refresh cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will beapparent upon consideration of the following detailed description, takenin conjunction with the accompanying drawings, in which like referencecharacters refer to like parts throughout, and in which:

FIG. 1 is a circuit diagram of a pair of DRAM cells;

FIG. 2 is a block diagram of a DRAM chip;

FIG. 3 is a diagram of a portion of a DRAM chip according to theinvention;

FIG. 4 is a diagram of an exemplary embodiment of a power bus switchaccording to the invention.

FIG. 5 is a circuit diagram of an exemplary embodiment of switch controlcircuitry according to the invention;

FIG. 6 is a timing diagram of an exemplary embodiment of switch controlinput and output signals according to the invention; and

FIG. 7 is a block diagram of a system that incorporates the invention.

DETAILED DESCRIPTION OF THE INVENTION

DRAMs are, in their simplest form, arrays of cells each including acapacitor for holding a charge and a transistor acting as a switch foraccessing the charge held in the capacitor. DRAM arrays are typicallyarranged in columns and rows. FIG. 1 shows a pair of DRAM cells 102.Each cell 102 is connected to a bit line 104 and a respective word line106 a or 106 b that controls the ON/OFF state of transistor 108. Bitline 104 is used to write information to or read information fromcapacitor 110 when the signal on word line 106 a or 106 b turnstransistor 108 ON (i.e., renders the transistor conductive).

A DRAM chip is a sophisticated device that can be thought of as havingtwo portions: the array, which includes a plurality of individual memorycells for storing data, and the peripherals, which include all of thecircuits needed to write information into and read information out ofthe array and to support the other functions of the chip.

FIG. 2 shows generally the functions on a DRAM chip. DRAM chip 200includes a memory array 212, read/write control logic 214, columndecoder 216, row decoder 218, refresh control logic 220, and senseamplifiers 222. Other peripherals (not shown in FIG. 2 to maintainclarity) include input and output data and address buffers, voltageregulators, redundancy circuits, test logic, and electrostatic discharge(ESD) devices. DRAM chip 200 also includes (also not shown in FIG. 2 forclarity) a plurality of data I/O pads, a plurality of power I/O pads, aplurality of ground I/O pads, power busing, and ground busing.

DRAMs require continuous power in order to retain their stored contents.If power to the DRAM is removed, all contents are lost. Furthermore,because capacitors normally discharge over time, DRAMs also requireperiodic refreshing to prevent loss of stored data. Refreshing of memorycontents is usually carried out row by row via refresh control logic220. Refresh operations lead to high current consumption. Moreover,refresh rates can cause the power needed to vary greatly. Providing thenecessary power may result in noise and other undesirable side effectswhen maximum power is not required, such as during normal read/writeoperations. In high speed SDRAMs (synchronous DRAMs), refresh rates ofabout 512 Meg per about 8 μ sec are known.

There are typically two types of refresh modes: AUTO refresh and SELFrefresh. AUTO refresh mode is typically used during normal operation ofthe computer or other electronic system that incorporates the DRAM. SELFrefresh mode is typically used when the computer or other electronicsystem goes into a sleep or low power mode (which still provides powerto the DRAM).

Note that while the arrays are being refreshed, read and writeoperations cannot be performed. If reads or writes are pending at thestart of a refresh cycle, one or more wait states occur until therefresh cycle completes.

Sense amplifiers 222 are the first element of the data read path and thelast element of the data write path. A sense amplifier 222 detects andrestores the capacitor voltage representing a data bit to itsappropriate value. This is done by first transferring the data bit fromthe capacitor of a memory cell to its associated bit line, measuring thepotential difference between the activated bit line and a reference linewhich may be an inactive bit line, and then adding or subtractingvoltage as needed. Sense amplifiers 222 thus sense and amplify the stateof data (i.e., a logical “1” or logical “0”). When a sense amplifier 222“fires” (i.e., becomes active), a large current drain occurs that causespower spikes on the power and ground buses.

To improve the performance of a DRAM (i.e., the speed with which readsand writes are made), a clock is used to control the transfer of dataand control signals within and to and from the DRAM. To improveperformance further, data and control signal transfers occur at both theleading and trailing edges of the clock signal. DRAMs operating in thismanner are known as a DDR (double data rate) SDRAMs.

Such high speed SDRAM operation requires control and data signalsynchronization among the various internal SDRAM devices in order thatthose devices correctly communicate each other. This is accomplished byusing one or more reference clock signals and synchronization controlcircuits, such as DLL circuits. DLL circuits output a delayed clocksignal that maintains a specific phase relationship with a referencesignal (which is typically the output of an oscillator). Theintentionally delayed signal ensures that data setup and hold times ofvarious devices such as flip-flops are met.

DLL circuits are sensitive to power and ground bus noise. Such noise isoften generated by row activations and read/write array activity. Forexample, when a row is activated during a read or write, large powerspikes occur on both the power and ground buses. Thus, to insulate DLLand other synchronization control circuits from such noise, separatepower and ground buses are provided to those circuits from designatedpower and ground pads that are not used to power the arrays. Separatepower and ground buses are provided to the arrays from other power andground pads.

FIG. 3 shows a portion of a DRAM chip in accordance with the invention.DRAM 300 includes an array 312, power pads 324, voltage regulators 326,array power bus 328, unregulated peripheral power bus 330, regulatedperipheral power bus 332, and ESD (electrostatic discharge) devices 334.DRAM 300 also includes ground buses, a number of I/O pads for data andcontrol signals, and DLL and other peripheral circuits, none of whichare shown in FIG. 3 for clarity.

Array 312 can be, for example, a 128 Meg array and DRAM 300 can include,for example, four such arrays 312. The number and size of arrays on aDRAM chip can vary, depending on the size of the chip, the desired chiplayout, the amount and type of peripherals required, and the technologyused to fabricate the arrays and peripheral devices. The invention isnot limited to a particular size or number of arrays.

The power, ground, and signal I/O pads are arranged down the center ofDRAM 300. However, the invention is not limited to this pad arrangement.Alternatively, for example, pads arranged around the periphery of thechip or in the central region of the chip are also within the scope ofthe invention. Power and ground pads receive externally suppliedvoltages for use on the DRAM chip. For example, in one embodiment of theinvention, ground pads typically receive about 0 volts while power pads324 receive about 2.5 volts, which is a common power supply voltage.Note, however, that the invention is not limited to a particular voltagevalue received at power pads 324.

Voltage regulators 326 are often included in DRAMs to derive a regulatedinternal supply voltage from the externally provided supply voltage.This is internal supply voltage is typically provided to the DRAM'smemory cell array and to the array's peripheral control circuits. In theembodiment of the invention mentioned above, the 2.5 volt externallyprovided supply voltage is regulated to preferably about 1.8 volts byregulators 326. Again, however, the invention is not limited to aparticular value of regulated voltage.

Because array activity (e.g., row activations and read and writeoperations) can produce noise that can affect the operation of thearray's peripheral control circuits, particularly the synchronizationcircuits, individual power pads 324 are dedicated for use by either thearray or the peripherals, but not both. Thus, as shown in FIG. 3, somepower pads 324 are dedicated for powering array 312 (denoted VccA) andothers are dedicated for powering the peripherals (denoted VccX). Notethat the selection and number of pads shown in FIG. 3 dedicated topowering the arrays and to powering the peripherals is merelyillustrative. Other numbers and arrangements of array and peripheralpowers pads are within the scope of the invention.

Note also that the invention is not limited to each power pad 324receiving the same voltage. For example, pads dedicated to powering thearrays may receive one voltage while pads dedicated to powering theperipherals may receive another voltage different than the first.

Similarly, note further that the invention is not limited to eachvoltage regulator 326 regulating the externally provided supply voltageto the same value. For example, voltage regulators regulating voltagefrom VccA pads may output one regulated voltage while voltage regulatorsregulating voltage from VccX pads may output another regulated voltagedifferent than the first.

VccA power pads 324 are connected to voltage regulators 326, which areconnected to array power bus 328. Array power bus 328 provides regulatedvoltage to array 312 sufficient to allow array 312 to performsubstantially all non-refresh operations, such as array reads andwrites. Array power bus 328 may be a single isolated bus connected tothe array 312 shown or, alternatively, array power bus 328 may connectto some or all other arrays 312 on DRAM 300 (assuming that DRAM 300 hasother arrays) and to some or all other voltage regulators 326 connectedto some or all other VccA power pads 324 on DRAM 300.

VccX power pads 324 are connected to unregulated peripheral power bus330, which provides the externally received voltage to ESD devices 334.

VccX power pads 324 are also connected to other voltage regulators 326,which are connected to regulated peripheral power bus 332. Regulatedperipheral power bus 332 provides regulated voltage to DLL and othersynchronization control circuits substantially free of power bus noisecaused by non-refresh array 312 activity. Other peripheral circuits,particularly those sensitive to power and ground noise, are preferablyalso connected to power bus 332, as well as any or all other peripheralsas desired or possible in view of chip design constraints and otherappropriate considerations. Similar to array power bus 328, regulatedperipheral power bus 332 may be a single isolated bus connected to theperipheral circuits associated with the array 312 shown or,alternatively, regulated peripheral power bus 332 may connect to some orall other peripherals associated with some or all other arrays 312 onDRAM 300 (assuming that DRAM 300 has other arrays) and to some or allother voltage regulators 326 connected to some or all other VccX powerpads 324 on DRAM 300.

Note that not all VccX power pads 324 on DRAM 300 need to be connectedto unregulated peripheral power bus 330, nor do all VccX power pads 324need to be coupled (via regulators 326) to regulated peripheral powerbus 332. Some VccX power pads 324 may be used exclusively forunregulated power bus 330, while others may be used exclusively forregulated power bus 332, while still others may be used for both.

Preferably, the voltage provided by unregulated peripheral power bus 330is higher than the voltage provided by regulated peripheral power bus332. In the embodiment of the invention mentioned above, unregulatedpower bus 330 provides preferably about 2.5 volts while regulated powerbus 332 provides preferably about 1.8 volts.

Thus, in accordance with the invention, VccA power pads 324 do notprovide power to DLL and other synchronization circuits, while VccXpower pads 324 do not provide power to the array during non-refreshmemory operations.

Although not shown, array 312 and the peripherals are also similarlyrespectively connected to separate ground buses that are connected torespective dedicated ground pads.

Note that the invention is not limited to the particular layout orarrangement of power buses shown. DRAM power buses in accordance withthe invention can be of other numbers, on one or more semiconductor chiplevels of power busing, and in other suitable arrangements.

Although the DLL and other synchronization circuits are substantiallyisolated from the adverse effects of power and ground noise generated bythe arrays, a disadvantage of having power pads dedicated to only theperipherals is the decreased number of power pads supplying the arraywith needed current during periods of high current demand, such asrefresh cycles. The current-supplying capability of each power pad islimited, and thus, the arrays may not be supplied with current ofsufficient magnitude to fully recharge the array cells' capacitors.

To compensate for this possible current deficiency during refreshcycles, DRAM 300 further includes switch control 336 and switch 338 inaccordance with the invention. Switch 338 provides a switchableconnection between array power bus 328 and unregulated peripheral powerbus 330. Switch 338 is coupled to array power bus 328 preferablyequidistantly between array power bus voltage regulators 326. Duringnon-refresh array operations, switch 338 remains open, isolating arraypower bus 328 from unregulated peripheral power bus 330. Preferably atthe start of each refresh operation, switch control 326 generates asignal pulse that closes switch 338 for a limited period of timepreferably much less than the duration of the refresh cycle. Switch 338preferably closes during activation of the sense amplifiers. The lengthof time that switch 338 should remain closed in accordance with theinvention depends on the array current load and the current handlingcapacity of switch 338. The array current load can vary depending on,for example, digit line capacitance, array voltage, or the number ofrows activated. For example, in one embodiment of the invention having aDRAM array of 128 Meg, an array power bus of about 1.8 volts, aperipheral power bus of about 2.5 volts, and a switch 338 having acurrent handling capacity of about 25 mA, a switch 338 closure time ofabout 10-12 nsec is sufficient to provide the array with additionalneeded current.

Switch control 336 preferably receives signals SAMP and ENABLE (in bothAUTO and SELF refresh modes). Signal ENABLE can be received from, forexample, refresh control 220 and indicates the start of a refresh cycle.Signal SAMP is received preferably just as one of sense amplifiers 222is about to fire in connection with that refresh cycle.

This temporary connection between array power bus 328 and peripheralpower bus 330 provides the arrays with additional current during arefresh cycle. The DLL and other synchronization circuits are notaffected by this temporary “shorting” of the two power buses because noreads or writes are performed during refresh, and thus nosynchronization of data and control signal transfers is required. Thelimited duration of the temporary power bus connection provided byswitch 338 preferably dampens out the current spike typically seen onthe array power bus during refresh.

Thus, in accordance with the invention, neither array power pads 324 norarray power bus 328 is directly connected on DRAM 300 to eitherperipheral power pads 324, unregulated peripheral power bus 330, orregulated peripheral power bus 332 other than as described with respectto switch 338 during refresh operations.

FIG. 4 shows an exemplary embodiment of a switch in accordance with theinvention. Switch 438 is preferably a field-effect transistor (FET), andis more preferably a p-channel FET. The gate of p-channel FET 438 isdriven by a signal pulse 440 from switch control 336. During array readsand writes, p-channel FET 438 is OFF (i.e., it does not conduct) and noconduction occurs between array power bus 328 and unregulated peripheralpower bus 330. At the start of a refresh cycle, the p-channel devicereceives a logical “0” signal pulse 440 from switch control 336, whichturns p-channel FET 438 ON (i.e., it conducts). This connects the 2.5volt peripheral power bus to the 1.8 volt array power bus, providing thearrays with additional current during a portion of the refresh cycle.

The p-channel device is sized to handle the current flow from peripheralpower bus 330 to array power bus 328. In an embodiment of the inventionhaving the following: a DRAM array of 128 Meg, an array power bus ofabout 1.8 volts, and a peripheral power bus of about 2.5 volts,p-channel FET 438 has a width of preferably about 750 microns and alength of about 1.2 microns. Channels of wider widths are usuallypreferable, and as is known in the art, p-channel FET 438 can be ofother sizes in accordance with current-handling requirements andfabrication technology constraints and considerations.

Alternatively, switch 338 can be other appropriate devices. For example,switch 338 can be an appropriately sized n-channel device. In general,the switch can also be any known device that can be activated (i.e.,closed) for a finite period of time by a signal pulse or equivalentmeans and that can handle current flow from the 2.5 volt bus to the 1.8volt bus for that finite period of time during the refresh cycle.

FIG. 5 shows an exemplary embodiment of switch control circuitry inaccordance with the invention. Switch control 536 includes NAND gate 544and inverters 542, 546, and 548. Inverter 542 receives an ENABLE signalas input, and NAND gate 544 receives an SAMP signal and the output ofinverter 542 as inputs. The ENABLE signal enables switch control 536 viaa logical 0 substantially upon the start of a refresh cycle. The ENABLEsignal may be generated by, for example, refresh control logic 220 (FIG.2) or by any other suitable source associated with DRAM refreshoperations either on the DRAM chip or received by the DRAM chip. TheSAMP signal preferably indicates via a logical 1 pulse that a senseamplifier is about to fire. Similarly, the SAMP signal may be generatedby, for example, refresh control logic 220 (FIG. 2) or by any othersuitable source associated with the firing of sense amplifiers duringDRAM refresh operations. This source may be either on the DRAM chip orreceived by the DRAM chip. The width of the SAMP logical 1 signal pulsedetermines the width of the output signal pulse at output node 550(i.e., signal pulse 440), and thus, the period of time that the switchallows conduction between array power bus 328 and peripheral power bus330. Output node 550 is coupled to at least one switch 338 or 438, andpreferably switch control 536 can drive more than one switch 338 or 438.

FIG. 6 shows a timing diagram 600 of an embodiment of DRAM signals andthe generation of a signal pulse 440 associated with a SELF refreshoperation in accordance with the invention. Signal CLK is a system clocksignal, signal CKE is a DRAM clock enable signal that allows SELFrefresh mode to execute asynchronously to the system clock, and signalCOMMAND is a refresh command signal. The DRAM enters SELF refresh modewhen signal CKE switches as shown at signal transition 652 and exitsself-refresh mode when signal CKE switches again. Time 654 representsthe propagation delay through switch control 536. The same relativetimings as shown in FIG. 6 also occur in AUTO refresh mode, except thatsignal CKE remains at logical 1.

Note that switch control 536 and timing diagram 600 are both merelyillustrative. Other switch control circuits that can produce signalpulses similar to or the same as signal pulse 440 in connection with arefresh operation can also be used. For example, in another embodimentof switch control circuitry, inverter 542 can be replaced with anon-inverting delay block (comprising any suitable logic circuit) andthe ENABLE signal can be replaced with a logically complementary signal.Similarly, in still another embodiment of switch control circuitry,inverters 546 and 548 can be replaced with a non-inverting delay blockhaving a propagation delay substantially the same as the combined delaythrough inverters 546 and 548.

The number of switches (and associated switch controls 336) implementedon a DRAM chip can vary depending on, for example, the size andorganization of the arrays and power buses on the chip, the currenthandling capability of each switch, the amount of current required forrefresh operations and the amount of current available for the arraypower busing, and the chip space available and its location relative tothe arrays and power buses. The number of switches implemented on a DRAMchip may additionally or alternatively depend on other considerations aswell.

FIG. 7 shows a system that incorporates the invention. System 700includes a plurality of DRAM chips 300, a processor 770, a memorycontroller 772, input devices 774, output devices 776, and optionalstorage devices 778. Data and control signals are transferred betweenprocessor 770 and memory controller 772 via bus 771. Similarly, data andcontrol signals are transferred between memory controller 772 and DRAMchips 300 via bus 773. Input devices 774 can include, for example, akeyboard, a mouse, a touch-pad display screen, or any other appropriatedevice that allows a user to enter information into system 700. Outputdevices 776 can include, for example, a video display unit, a printer,or any other appropriate device capable of providing output data to auser. Note that input devices 774 and output devices 776 canalternatively be a single input/output device. Storage devices 778 caninclude, for example, one or more disk or tape drives.

Note that the invention is not limited to DRAM chips, but is applicableto other integrated circuit chips having a circuit or group of circuitssensitive to power or ground noise generated by a second circuit orgroup of circuits in which the second circuit or group also requirestemporary increases in current for periodically or occasionallyperformed functions or operations.

Thus it is seen that DRAM power bus control is provided to isolateperipheral circuits from power bus noise generated by memory arraysduring reads and writes, while still providing the arrays withsufficient current during refresh cycles. One skilled in the art willappreciate that the invention can be practiced by other than thedescribed embodiments, which are presented for purposes of illustrationand not of limitation, and the invention is limited only by the claimswhich follow.

1. A method for controlling power distribution on an integrated circuitchip having dynamic random access memory, said dynamic random accessmemory comprising at least one array operative to be read from andwritten to and being periodically refreshed in order to maintain storedvalues, said method comprising: powering a delay lock loop circuit;separately powering reads and writes of said array; and increasing powerof said powering reads and writes for a finite period of time duringsaid periodic refreshing of said array.
 2. The method of claim 1wherein: said powering said delay lock loop circuit comprises poweringwith a first voltage; said separately powering reads and writescomprises powering with a second voltage; and said increasing powercomprises powering with a third voltage higher than said second voltage.3. The method of claim 2 further comprising: receiving said firstvoltage from a first integrated circuit chip power pad; and receivingsaid second voltage from a second integrated circuit chip power pad. 4.The method of claim 3 further comprising receiving said third voltagefrom said second integrated circuit chip power pad and a thirdintegrated circuit chip power pad for said finite period of time duringsaid periodic refreshing of said array.
 5. The method of claim 3 furthercomprising receiving said third voltage from said first integratedcircuit chip power pad and said second integrated circuit chip power padfor said finite period of time during said periodic refreshing of saidarray.